open customer/main/product.tpl and look for
Code:
{if $product.price ne 0}
<font class=ProductDetailsTitle>{include file="currency.tpl" value=$product.price}</font><font class=MarketPrice> {include file="customer/main/alter_currency_value.tpl" alter_currency_value=$product.price}</font>{if $config.Taxes.use_vat eq "Y" and $product.vat gt "0"}, {$lng.lbl_including_vat} {$product.vat}%{/if}
{else}
<input type=text size=7 name=price>
{/if}
change to
Code:
{if $product.price ne 0}
<font class=ProductDetailsTitle>{include file="currency.tpl" value=$product.price}</font><font class=MarketPrice> {include file="customer/main/alter_currency_value.tpl" alter_currency_value=$product.price}</font>{if $config.Taxes.use_vat eq "Y" and $product.vat gt "0"}, {$lng.lbl_including_vat} {$product.vat}%{/if}
{else}
Call For Price
{/if}
you may have to do a similar thing to products.tpl and products_t.tpl but this will give you an idea of what to look out for