The raw data: open and response rates.
Across roughly 240,000 dunning messages we have visibility into across SMBs in WhatsApp-dominant markets in the last 18 months: WhatsApp open rate is 88 percent within 24 hours. Email open rate is 31 percent within 24 hours in the same markets. Response rate (any reply, not just payment): WhatsApp 22 percent, email 6 percent. In US and UK B2B markets, those numbers invert — email open rates run higher (45-60 percent for transactional emails) and WhatsApp is rarely used at all.
But open rate is not the same as paid rate. WhatsApp gets read but does not always lead to payment because the message is easier to dismiss. Email feels more formal and creates more urgency for B2B customers. The right answer is not "use WhatsApp" or "use email" — it is "use the right channel for the right region, customer segment, and stage."
Channel by stage of the dunning ladder.
In a WhatsApp-strong region (LATAM, India, southern Europe, Middle East, parts of southeast Asia): Stage 1 (1 day past due, friendly nudge): email. Low-stakes, want to record it, not pushy. Stage 2 (7 days): WhatsApp. Most reads come from here. Reply rate highest. Stage 3 (21 days): both. Email for record, WhatsApp for read. Stage 4 (45 days): WhatsApp first, email backup. Stage 5 (60 days): email plus phone call.
In a US / UK B2B-style market: Stage 1: email. Stage 2: email follow-up with personalised subject line. Stage 3: email plus a phone call. Stage 4: email plus phone plus optional SMS. Stage 5: formal letter plus phone. WhatsApp is rarely the right channel for B2B AP departments in these markets. SMS plays a useful role in Australia and Northern Europe for stages 2-3 where SMS open rates rival WhatsApp.
- WhatsApp-strong markets: WhatsApp for stages 2-4, email for record
- US/UK B2B: email primary, phone for escalation
- AU / Northern Europe: SMS plays the role WhatsApp plays elsewhere
- Phone for all stage 5 escalations
- Per-customer channel preferences override defaults
Customer segment matters.
B2B corporate customers (banks, multinationals, larger SMBs): email-first regardless of region. Their AP processes run on email and a WhatsApp / SMS message often goes to the wrong person (sales rep instead of accounts payable). B2B SMB-to-SMB: WhatsApp-first in WhatsApp-dominant regions, email-first elsewhere. Owner-to-owner conversation is faster on WhatsApp and often reaches the decision-maker directly.
B2C customers (retail credit, service customers): in WhatsApp-strong markets, WhatsApp throughout with email only for stage 1 and stage 5. In US / UK markets, SMS and email work. The relationship is personal, the amounts are typically smaller. Adjust based on your customer mix; the same business might use different channel mixes for different customer segments and geographies.
Tone differences between channels.
WhatsApp / SMS tone is conversational. Use first person ("Hope you are well, just a quick reminder..."), avoid corporate language, keep messages short (3-4 lines max for WhatsApp, under 160 characters for SMS). Include the invoice number and amount but skip the full legal language. Email tone is formal. Full sentences, complete header information, attached PDF of the invoice, signature block. Same content, different presentation.
A common mistake is sending the same template on both channels. The email looks fine but the WhatsApp version feels robotic and reduces response rate. Maintain separate templates per stage per channel. The investment in writing them once pays off across thousands of messages.
Worked example: a building materials supplier.
A building materials supplier in São Paulo, R$1.6M monthly revenue, 400+ active customers. Pre-automation: email-only dunning, average DSO 58 days, ~R$3.2M in 60+ day AR. Implemented Nonari's combined channel dunning (WhatsApp + email) over 4 months.
Result by month 4: email-only customers (those who opted out of WhatsApp): DSO 51 days. WhatsApp+email customers: DSO 38 days. The WhatsApp+email cohort had 47 percent fewer customers in 60+ day buckets. Total cash freed: R$2.1M. The cost was modest: WhatsApp Business API messaging fees of about R$1,200 monthly.
When WhatsApp / SMS goes wrong.
Three failure modes. One: too many messages, customer mutes or blocks. Solution: respect cadence (max 1 dunning message per week per invoice), and give an opt-out link. Two: messages from a personal number, not a business number. Customers stop trusting it. Solution: WhatsApp Business API number, not a staff member's personal number. Three: tone too casual, looks unprofessional. Solution: templates reviewed by the owner, not improvised.
A fourth subtle failure: messages with no clear payment link. The customer reads, intends to pay, gets distracted, forgets. Solution: every message includes a clickable payment link or clear next-step ("reply YES and we will send the bank details"). Friction kills conversion.
How Nonari handles channel routing.
Per-customer channel preferences are configurable. AR aging buckets trigger the dunning sequence. Each stage has a per-channel template. WhatsApp messages go through the WhatsApp Business API with delivery and read receipts logged. SMS goes through Twilio or local providers. Email goes through SMTP with open tracking. Replies route to the AR inbox for human handling.
The dashboard shows per-customer dunning history (which messages were sent on which channel, with which results) and per-channel effectiveness metrics (read rate, response rate, payment rate). Tuning the cadence is easy because the data is right there. Most SMBs find their right mix within 60-90 days of using the system.