German tanks of WWII were not equipped with telephones that allowed infantry to communicate with the tank's crew. This is partially where you get the archetype of the German tank commander standing proudly in his hatch; he did this partially to see the battlefield better, but also to communicate with infantry that were accompanying his tank. If the tank was buttoned up, infantry would need to rap on the side with a rifle butt or make a loud noise to alert the commander of something. A rather hilarious story of American infantry (before Shermans had field phones) involved them beating on the buttoned up Sherman with a rifle butt, standing in front of it and waving their arms, and even covering the driver's periscope with their hands to get his attention!

US tanks, (in this case a Sherman) on the other hand, were sometimes equipped with a telephone that allowed infantry to squat safely behind or beneath the tank. It was not a standard item at first and was kept in an ammunition can welded to the back of the tank. Later tanks had the phone as a standard item.

This ad-hoc phone was first trialed during the Normandy campaign and remained in use until the end of the war. Here it can be seen in Germany in December 1944. It is in the ammunition can below the cooking pot.

http://imgur.com/JQ1dahS

The standardized phone, the RC-298 Interphone Extension Kit, didn't come into use until very late in WWII and saw no service in that war. According to Sherman: Design and Development, it was first added to Shermans on the production line in the third quarter of 1945, among the last Shermans built. It was retrofitted to many earlier Shermans and saw service in Korea. Here it is on an M4A3(76)W HVSS. It is in the box below and to the left of the right-hand tail light. You would pull the handle to open the box.

http://imgur.com/71rO28A

Sources:

Sherman: Design and Development, by Patrick Stansell and Kurt Laughlin