The trouble with hacked together technology

I have been sending SMS messages from my mobile, using a local (Vodafone) SIM here in Australia, to a friend who is registered with the O2 network in the UK but who has been holidaying in Spain and whose phone is thus roaming. Some of them have been arriving instantly, and some have been taking as much as several days. I have also been sending messages to a few people in the UK. Some are getting through, but I am unsure about others. It could be that there is some latency there, but it could also just be that my friends are slack in replying.