I think SIP is definitely the way to go. A while back I was looking for a provider in the EU and found voxbeam.com. Their servers are in Amsterdam and have go

I think SIP is definitely the way to go. A while back I was looking for a provider in the EU and found voxbeam.com. Their servers are in Amsterdam and have good connectivity. Traceroute to sip.voxbeam.com to get an idea of the latency you'll see. They also do point-to-point audio so if they hand your call to another provider's server, they get out of the media stream (reduces latency). One potential issue with SIP is that you might have issues behind some firewalls. Not sure how big a deal that is in practice while traveling (I think its probably fine). If not it might be useful to use Asterisk+IAX. I'm not sure what Android clients exist for IAX. I ended up building an Asterisk box on EC2 EU-WEST-1 (Ireland), which, at ~20ms from sip.voxbeam.com, doesn't add a ton of latency. It works fine on a Micro instance, which is free for a year if you don't already have an AWS account.