Basically, when sending through a GSM modem or GSM phone, you should use the format:
This format is accepted by the majority of all telecom operators. In case this format is not working, you should try the number without country code.
Be aware of the different requirements of service providers. Some allow sending SMS messages to foreign GSM phones, others don't. Some providers require a country code in the number, even when sending to an SMS number in the same country. Others also require the international dialing number.
Other providers do not accept the country code and international dialing code.
A major problem is, that SMS service providers not always generate an error when the recipient format is wrong. Therefore, you must always find out the recipient format and test it.
Example:If you want to send an SMS message to 12345678 in the UK, you should try the following numbers: