Push-to-talk allows handsets to be--a feature mobile phone operators believe will increase their revenue by encouraging people to talk more.
The three companies will conduct tests to make their technologies compatible, enabling customers of different operators to talk to one another--something that is essential if push-to-talk is to penetrate the mass market.
Many phone companies, including Siemens and market leader Nokia, are part of the Open Mobile Alliance, which aims to develop unified, open standards for various kinds of messaging. But the phone companies are still pursuing different paths.
Nokia last month announced a separate interoperability cooperation deal with Samsung, whom it supplies with push-to-talk technology.
A Siemens representative said the company is still committed to a single push-to-talk standard and expects the alliance with Motorola and Ericsson to expand.
"Nokia was invited to participate, but they decided not to at this stage,'' the representative said. "We are still promoting an open standard.''
He added that other companies want to join the Siemens alliance, without giving further details.
On Wednesday, Siemens plans to show off its first push-to-talk-enabled phone at Germany's CeBit telecommunication and electronics trade show.
The handset, a version of its CX65 phone launched three weeks ago, will be on the market in the second quarter.