Yet another ad based voip service that has recently launched is Plumble.  Plumble is current in it’s beta mode with restricted access to beta members, however, it is possible to make free international calls with Plumble without signing up.

Steps to make free international calls with Plumble

1) Dial 818-742-0110 (Number from the US)

2) Listen to various short ads.

3) Dial your free call

4) Keep in mind that at the moment you can only call the USA and Canada for free, which should include mobile phones.