📤 Character encoding when sending SMS (PDU)

📦 UTF-8, ISO & Compatibility Issues with Mobile Devices

Background: SMS in PDU Format

Our gateways send SMS messages using the PDU format with UTF-8 character encoding by default.
This configuration is part of the device’s internal settings.

🔍 Behavior in Mobile Networks (e.g., Telekom Germany)

According to feedback from Telekom, the mobile network treats every SMS as “GSM 7-bit alphabet”, regardless of the gateway’s configured encoding.
→ This can cause display issues on certain devices.

⚠️ Issue: SMS Are Not Displayed Correctly on the Recipient Device

A customer reported that received SMS messages were unreadable – for example, containing incorrect special characters or completely garbled text.

🔎 Analysis revealed:

• Root cause: Use of UTF-8 encoding

• Solution: Switched to ISO encoding (ISO-8859-1)
  → This format is more compatible with older GSM handsets

After switching, all SMS messages were correctly received and displayed.

🛠️ Temporary Technical Solution

We can offer you an alternative character encoding configuration.
However, this setting must be manually applied via SSH access to the gateway.

This requires a remote session using TeamViewer or a similar remote support tool.

⚠️ Important:
A valid support contract that includes remote sessions is required.